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Telescopes by Sir Issac Newton
Years ago, one of the UK observatories had a replica of a telescope made by Sir Isaac Newton in the late 17th century.
It was marked “For use in an emergency,” jokingly suggesting that if the large telescope failed, astronomers could use it.
In fact, that little telescope was a change of astronomical game in its day, and is now the standard design for most optical telescopes for astronomy, whether in observatories or backyards.
At some point in our young lives, most of us were given toy binoculars. They were usually made of brightly colored plastic, with plastic lenses. The binoculars worked, but where there was a clear cross between light and dark areas, like the edge of a rooftop with the sky in the background, we saw colors: red on one side and blue on the other.
This is an effect known as chromatic aberration. It is due to the different colors of light that pass through the lenses at different speeds. That means they focus on different places. Today, using composite lenses made of different types of glass, this problem can be corrected. In Newton’s time, however, chromatic aberration was a major problem.
Newton’s idea was that if light collected by a telescope did not have to pass through glass on its way to becoming an image, chromatic aberration would not occur. So he used a concave mirror.
With a lens, the light focuses behind the lens, forming an image. With a concave mirror, the light is focused at a point in front of the mirror, which puts the image in a very awkward place.
Newton added another small mirror, near the focal point. This sent the converging light, before it actually formed the image, out of the side of the telescope where the image could be easily seen, at the expense of having this small mirror blocking some of the light.
By placing the reflective coating on the front face of the mirror, no light passed through the glass and all colors were sent to the same spotlight. There was no chromatic aberration. The concave mirror of Newton’s telescope was not made of glass at all. He used a metal alloy called a “speculum”. After all, the material of the mirror should not be transparent. Today, mirrors are made of special glass or ceramic.
Another great advantage of reflecting telescopes is that the mirrors can be made very large. Because light must pass through the lenses as freely as possible, the only place where a lens can be supported is around the edge.
This means that there is a limit to the size that can be made before it is distorted by its own weight or even broken. It is easy to bear a mirror. We can put as much support structure or other hardware as we like behind it, because the light is not there. This brings us to another advantage. The lenses should be at the top of a telescope. Mirrors go to the lower end, where it is easier to handle heavy weights, the telescope never weighs on top, making it safer and easier to control.
The largest mirror we can make is about five feet in diameter. As the diameter increases, so should the thickness, in order to keep it in shape as the telescope points in different directions.
Today we discovered how to make much bigger mirrors. We can make them with unique pieces of relatively thin material or with many hexagonal panels. Behind the mirror are many computer-controlled actuators, which continuously adjust the mirror to keep it in shape. Mirrors about 10 meters in diameter are now being used and a telescope with a 30 meter mirror is being built.
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• On June 21, the Sun will reach its northernmost point in its annual travels, and at noon it will be higher in the sky than at any other time of the year. This will give us the day with the longest light period.
• Mercury hides under the glow of dawn, with Venus taller and brighter. To the right of Venus are Mars and Jupiter together, and then Saturn.
• The Moon will be new on June 28.
This article is written by or on behalf of an outsourced columnist and does not necessarily reflect Castanet’s views.